Linhenykus: the very model of modern Mongol, errr, alvarezsaur.

Yep it’s another new paper and another new alvarezsaur. Way back when with Xixianykus I mentioned that there were more to come. One of these was Haplocheirus and now Linhenykus is on the scene. As with Linheraptor this was another find by Johan Choiniere when out with Mike Pittman, and as I recall was found just the day before that cracking dromaeosaur.

This specimen had eroded out of a nodule, rather unusually for Bayan Mandahu, but the bones were in superb condition and aside from the head and tail, was pretty much complete. It’s a tiny thing, the living animal would probably have been able to sand comfortably in the palm of your hand, but even so it has one rather unusual feature – it has only one finger.

Linhenykus elelments. Modified from Xu et al., 2011

Now the mononykines (more properly parvicursorines) the sub-group of alvarezsaurs to which this belongs are, if you know your languages, defined by their single large claw on the hand (mono = one, nykus = claw). But this is in conjunction with either one or two smaller fingers on the hand, not too dissimilar to Cyclopes, the pygmy anteater. Linhenykus however has just one functional finger, the first non-avian dinosaur known with this feature. There is a second metacarpal (one of the palm bones) preserved, but this has no phalanges attached and indeed tapers to a point distally showing that the hand really didn’t have any other fingers. This is a one fingered theropod. Actually the presence of that second metacarpal is rather handy, without it it would be easy to conclude that the rest of the hand was just missing, but being present and in a condition where it would not support any phalanges shows that we do have the whole hand and this was genuinely one fingered.

Simple alvarezsaur phylogeny and hand evolution. From Xu et al., 2011

Interestingly, Linhenykus is not an especially derived alvarezsaur despite the obviously highly modified hand even compared to other alvarezsaurs (see the hands and phylogenies image). Its main claw is not as big as later relatives so it is derived in terms of digit reduction but basal in terms of claw development and robusticity. This suggests at least the possibility that the other digits were effectively vestigial in derived alvarezsaurs. If they were non-functional then they would be under neutral selection, they might be lost, or hang around in various lineages and their reduction or loss could be down to nothing more than chance. This is of course a working hypothesis and it will be very interesting to see if other alvarezsaurs turn up showing similar variability in the state of their non-specialised digits.

Good spot and yes you are! It’s because Linhenykus is at the base of a clade and the figure is jsut using Shuvuuia to represent that. Which in hindsight, it doesn’t really do very well, but the other figures in the paper should make it more obvious.

I thought that might be it, but even then, it looks more derived (i.e., more changed from the ancestral condition) than its sister group, however more speciose that sister group is. (Especially when you consider that its greater, er, speciosity[?] could easily be an artifact of our incomplete knowledge of fossil taxa.)

Well if you go by that figure alone, yes. Which is part of the point of inferring vestigiality / non-functionality. Based on all the information this is a relatively basal form but the fingers look derived. So Linhenykus lost the other fingers when later, more derived taxa, did not.

In my bit of press copy I said it was mole-like. We are looking at the same fossil, but you’re not looking at the right bits Ed, or perhaps not in the right way. There are a bunch of digging adaptations in derived alvarezsaurs which include (off the top of my head):

All of these are seen in, if not Linhenykus (thought incompleteness), then parvicrusorines in general. All of these are also in things like moles, aardvarks, anteaters and yes, armadillos. There are some others which can be associated with digging like the enlarged sternum too which are not necessarily in the mammals.

I can see the digging adaptations- I certainly can’t see any other reason for claws and musculature like that. But with arms that short, mounted on a fairly normal shaped small dinosaur I find it very hard to see how it actually used its tiny arms.

Regardless of personal incredulity its a lovely fossil. Well done on the paper!

Dave, you only mention Shuvuuia here, when considering having only one functional digit. My understanding is that the holotypic manus for Mononykus olecranus also lacks any clear indication (or preservation) of additional digits, thus implying that it, too, lacked additional functional digits. How does this fly, if true, with the premise of the paper?

Nitpick- I notice the abstract says Linhenykus is “the most
basal parvicursorine”, but since Choiniere et al. (2010) defined Parvicursorinae as (Mononykus+Parvicursor), this is impossible. If it’s more basal than Mononykus and Parvicursor, it’s not a parvicursorine.

I still think using the II-III-IV model for tetanurine hands does more to confuse the literature than it helps, since no one else follows it.

Well as ever, with the digit thing, regardless if other people do or do not follow us, until the II-IV hypothesis is properly tested and refuted it would remain a valid hypothesis. Even if *we* didn’t follow it, I think we would be obliged to say in the paper that we are using I-III and not II-IV.

In the meantime, we’re kind of stuck with it. As a rough analogy, there are very few people out there who still consider Tarbosaurus to be Tyrannosaurs, but they still say explicitly in their papers that they are following this taxonomic system. People may roll their eyes, but no one gives them any hassle for it *becuase* they are making their intentions clear and it’s easy to track what they have done through the references they cite.

In that case though, there’s been years of research and loads of papers commenting on the issue. In the case of the new material from Limusaurus there’s been, what, one pseudo-published ‘announcentment’. I’d argue that we are still very much in the earliest days of this hypothesis and to pretend it doesn’t exist is the wrong way to go.

Very cool, but of course I have a possible nitpick about the otherwise awesome artwork accompanying the press articles. Alvarezsaurids are traditionally depicted with short tails and that continues here, but a) wasn’t this based on the old assumption that they were closer to pygostylians than Archaeopteryx and b) don’t newer specimens (Shuvuuia?) with complete tails show that they weren’t notably shorter than other theropods?

It should be noted that I restored the tail from said specimen (MPD 100/120, aka “the Suzuki specimen”) while spacing breaks in the tail according to apparent spacing in the fossil; even collapsing all spaces to account for preserved caudal material, the tail is fairly long, and indicates that preserved tail length in Mononykus olecranus (GI 107/6, holotype) is less than 1/2 preserved length. As Matt says, this is been universally despicted and continues to be so in both Mongolian and American exhibits.

Actually, the preserved tail length of Mononykus is a single proximal caudal. The caudal vertebrae used in its original reconstruction that suggested a short tail are those of IGM 100/99, which is now thought to be Shuvuuia or closer to Parvicursor.

While I’m here, in response to one of Hone’s listed fossorial characters- how are parvicursorine vertebrae more resistant to torsion than most theropods’? If anything, the lack of hyposphene-hypantrum articulations and opisthocoely would suggest more flexibility to me.

Similarly, the steeply angled zygapophyseal facets of Xixianykus would have contributed to holding the trunk rigid and reducing the potential for lateral undulation. Such angulation is unusual among theropods, although alternative stabilizing mechanisms such as hyposphene-hypantrum articulations (e.g., Sinraptor, Currie & Zhao 1993) and transversely concavo-convex zygapophyseal articulations (e.g., Deinonychus, Ostrom 1969) have been reported in various taxa. In the case of Xixianykus, the functional interpretation of the angled zygapophyses is complicated by the fact that the parvicursorine forelimb shows probable adaptations for digging into insect nests (Senter 2005; Longrich & Currie 2009). It is possible that the primary selective value of the angled zygapophyses lay in stabilization of the trunk during digging, rather than during running.

The holotype carpometacarpus lacks preserved digits; in their description of Shuvuuia, the authors ascribed that two little “nodes” on the external edge of the first digit (positional homology, rather than identity) cotylus would correspond to metacarpals and that they could, apparently, bear digits; this, thus, would agree with the morphology in Shuvuuia, which includes the digits in multiple specimens. This has not been affirmed as there appears to only be one specimen of said taxon now.

Tiny spurs and the lack of any clear indication of phalangeal condyles on the carpometacarpus of Ceratonykus, and Albertonykus lacks the relevant material to compare; it is generally assumed that Patagonykus is too incomplete to affirm or deny any additional digits.

I’d definately say though that the morphology of the manus certainly does differ from other species it can be compared to, and clearly shows a diminished if not completely absent third(fourth?) metacarpal, which is — as you say — unique.

It is heading that way. I think we have run out at the IVPP for now, though 3 in two years is pretty good. There is some more material, but no new taxa that I’m aware of. It’s been a good few years for alvarezsaurs and I’d not be surprised if there were more coming out of China or South America in the near future.

Not being able to see the paper, I would like some clarification on size. Is this taxa smaller than Parvicursor? And is it clear whether it is a juvenile or not?

While the painting is well-composed, I have to say for some reason the scale seems off. Of course, you couldn’t draw a human next to it, but something about it (maybe the size of the dune waves) makes it seem ornithomimid sized.

I had the same impression of the picture making it look big; I think that it’s largely because the viewer is kind of looking *up* at it – so since we naturally assume a human eye level it looks weird. The viewpoint would have to be very close to the ground to look like it does in the picture.

The termite eating hypothesis doesn’t make a lot of sense to me. The arms are so short, that the animal would have been scratching “blind”, and would have been unable to dig to any depth into nests. The neck is very long which would have very awkward for getting the head down in between the claws – right up against the chest – to lap up any termites/ants in this nest-ripping scenario. All modern ant/termite eaters are exceptionally heavily built, slow animals whose arms and claws are very large in relation to the body. Whilst we shouldn’t necessarily expect extinct animals to be exact analogues of modern equivalents, the fact that this is an exceptionally gracile creature is not a good sign for a termitivore hypothesis. There may be no modern analogue for these things. I think something else is going on – maybe like the abelisaurs, the arms were becoming highly reduced because they weren’t useful – but then acquired a secondary utility (brooding, mating, intraspecific competition or display?) requiring strengthening of the vestigial stumps.

The arms are very short, but then so are those of things like pygmy anteaters. Most things dig blind in any case becuase they close their eyes to avoid getting dirt in them. I’d see the long neck as an advantage, not a disadvantage – once the nest is open, the head and neck can get in there. A giant anteater or aardvark or pangolin has a long snout well in front and above the claws but this is not an issue there at all.

I’d disagree about ant eating animals being slow. The tamandura and giant anteater are both surprisingly quick and the aardvark and aardwolf both travel long distances on any given day. Most of these animals are heavily built because they dig down seriously (they can or do burrow, or may deep excavations) but as I describe above, alvarezsaurs do have a long list of digging adaptations and they do have a short and powerful chest and arms, just mounted on long legs. This is not an especially gracile animal in terms of the functional morphology for digging. I think you are confusing digging (in this case, specifically hook and pull digging [not scratch digging as I mistakenly said earlier]) with burrowing.

If these were non functional then they would ultimately be lost (as the fingers were). If there is another function no one has suggested one and this would have to ignore aqll the actually strong evidence that these things could, and did, dig.

The male brush turkey of Queensland, Australia, builds vegetation piles for an acquiescent female to lay her eggs in, then tends the temperature by adjusting the pile. Maybe this is the magic analogy for the hands of alvarezsaurs!

Sorry but that doesn’t really fit the available info. These arms are specialised for breaking into relatively tough things, not simple dragging or unearthing mounds of vegeatation. You’d want hands that are more shovel-shaped for that, not terminating at a point.

I really struggle to picture this animal feeding on ants or termites. How would it look? First, it would be restricted to above-ground nests – as you point out, these could conceivably be ripping tools but they’re not much use for actually digging. So we’re talking about nest mounds or rotting logs. Then, it would need to stand tall and sort of clasp the nest to its chest. Or could it have straddled a rotting log?? And then what – squeeze to pierce in a pincer movement, and pull? Could it have scratched downwards with any great force? It just doesn’t seem right. It would have taken an age to get into a termite mound, the tools just don’t seem fitted to the task.
But even if this were feasible (stranger things have happened), it’s the range of species that makes me skeptical. Other Alvarezsaurs are similarly stunted. In a radiation of termitivores, why didn’t they develop longer arms to rip nests to pieces, or to dig up buried nests (surely a very large, unattainable resource for these creatures and therefore an ecological anomaly in evolutionary time)? The adaptations you list certainly suggest that the arms were strong, and the torso somewhat rigid, but these features could equally be convergent based on an unrelated “pincering” behaviour. An equally weak case could be made that these were egg-crackers specialising on tough dinosaur eggs (another abundant resource?). i’m not going to defend this, I’m just saying that these don’t seem like feeding implements.
If not – then what? My best shot in the dark is that the arms were used in intraspecific competition. Little insectivores (?) with no great use for arms could have needed to keep their mass to a minimum, but kept/developed small, strong arms for e.g. ritualised wrestling, a bit like Komodo dragons. The animals might have stood chest to chest and used their arms to clasp and twist their opponents off their feet.
Is this a testable hypothesis?

Well you are welcome to disagree but I don’t think you have looked at the evidence well enough and are coming up with problems which don’t necessarily exist. Like I have said, there are anteating animals with short arms (notably pygmy anteaters). If you are a small animal like the avaergae alvarezsaur then you don’t necessarily need to dig deep into a nest, merely break open the surface and feed on a few termites. Once you break and ant or termite nest they rush out to defend and / or repair it, so they are right there on the surface for you – you don’t *have* to get right in. They might well be eating other insect and things like beetle larvae as well which would help out.

I do agree that getting the claws to engage could be problem in cases, but it is not necessarily a big one and there are parallels in other living ant eating taxa.

“In a radiation of termitivores, why didn’t they develop longer arms to rip nests to pieces, or to dig up buried nests”

In this case it’s because if you do have longer arms you will lose the power. You can;’t have a very strong and long lever (for a given musculature). You can have reach or power but not both and presumably power was the real issue. This is a common feature of these digging adaptations and as I have said and is quite well studied, specifically for hook and pull digging. The arms are not stunted, they are supposed to be this shape and size.

You could test your hypothesis but I think I can fail it already I’m afraid. If they were fighting and twisting each other then the arms would have to move in a rather different way to the way that they do move. These things could pull in and down, but not especially grasp and twist. If they were doing this to each other then the power would still come in the arms, but would not necessarily need to be delivered through the fingers so one would expect to see more and bigger fingers rather than just one and the others reducing. You might also expect to see this only in males or to see evidence of injuries in the shoulders and sterna of alvarezsaurs and there’s noting for any of this.

Sorry but digging is well supported by lots of good evidence and consistent characteristics.

But they are not long arms and they are on a very small animal. They would have the same kinds of problems that you assert would affect small alvarezsaurs – a small animal with short arms trying to engage a nest or wood. Plus they also have to compromise since these are climbing quadrupeds, wheres alvarezsaurs can actually specialise more since their forelimbs are not used in locomotion.

Well it looks like we’ll have to agree to disagree! I could go along with the digging hypothesis for some of the other alverezsaurs because it seemed to be the only one that could possibly explain the morphology. Looking at an animal like Xixianykus you can *just about* see it scratching away at a log. But Linheraptor with its minute pincers really throws serious doubt on this theory for me. As a simple-minded ecologist I tried an experiment yesterday walking back through the woods – thankfully there were no passers-by – I had a go at scratching at a standing snag with my index fingers next to my nipples in my best approximation of a Linheraptor style . I know we’re not built much like a Linheraptor but you should try it – it feels completely inadequate. You can’t see what you’re doing, you get no purchase on the wood, and your chest gets in the way.
I think that before we accept a digging hypothesis, we need to look a lot harder at the evidence to the contrary.
For the intraspecific competition hypothesis, I think it’s true that the reduction in finger number isn’t very consistent with grappling; but I wouldn’t necessarily expect to have found evidence of injuries – the specimens so far are very few and fragmentary, and ritual combat in small animals very rarely results in serious injuries (scratches, yes, bone damage, no).

“But Linheraptor [I assume you mean Linhenykus] with its minute pincers really throws serious doubt on this theory for me.”

But it’s general morphology is absolutely identical to other alvarezsaurs! The only change is the reduction of the non-functional fingers. So if you accept this for the others, you have to accept that for this too.

“You can’t see what you’re doing, you get no purchase on the wood, and your chest gets in the way.”

As I have already explained not seeing what you are doing is not an issue here as it is true of all digging animals. And the reach is not much of an issue when you are only a foot tall. And your fingers are not powerful levers with massive claws on the end. This is like trying to pick up a pencil with your top lip and nose and claiming that elephants can’t have used their trunks. It’s not a reasonable analogy.

“I think that before we accept a digging hypothesis, we need to look a lot harder at the evidence to the contrary.”

Perhaps. but you are, i think, ignoring all the evidence in favour. Go and read the literature on digging adapatations. Alvarezsaurs have everything in common with other diggers, and none of these features show up in animals that do not dig.

“I think it’s true that the reduction in finger number isn’t very consistent with grappling;”

And as I said the entire orinetation and motion of the arms. Look all I can say is there is a lot of stuff on this and much as I enjoy writing the blog, I’m not here to sit and regurgitate a bunch of papers and book chapters to you in detail to explain my point. That’s up to you to go and read.

Hi Dave, I would disagree with many of the behavioural and ecological points you state in favour and I should probably start with the truly extraordinary statement:

“Alvarezsaurs have everything in common with other diggers!”

but when we start getting into point-by-point rebuttal it’s time to smile and call it a day. There is no doubt you know the morphology, literature and specimens much better than I do, but if that gives you such unshakeable confidence, then I think you’ve gotten so close to the details that you’re not seeing the wood for the trees 🙂 Good chatting with you, and I do hope more specimens will emerge to shed further light on this strange radiation!

“There is no doubt you know the morphology, literature and specimens much better than I do, but if that gives you such unshakeable confidence, then I think you’ve gotten so close to the details that you’re not seeing the wood for the trees”

I’m not trying to be petty but I’m genuinely not sure if that is supposed to be sarcastic or dismissive or not, (despite the smiley). All I will say is that you seem to be saying that *because* I have actually read the literature, analysed the behaviour, compared the anatomy of digging species and alvarezsaurs and so on, that I can’t understand the problem properly. That because I have evaluated and assessed the evidence in detail, I have come to the wrong conclusion. Sorry, but that is just wrong.

I don’t mean to be either sarcastic or dismissive so please don’t read too much into my comment (imparting the wrong tone is a real danger of blog comments). You’ve thought about it and worked on it a great deal, I haven’t read all your papers, and you’re probably dead right. So take it as a personal failing on my part, that as an ecologist I simply can’t visualise this (or the other alvarezsaurs, but particularly those with the most reduced arms) feeding efficiently as a termite scratcher, and I just wonder if there isn’t a behaviour or an ecological niche we just haven’t thought of. I will indeed read the detailed papers and see if I can eliminate in my own mind the possibility of morphological convergence. If you’re right, well, natural selection is an even stranger designer than I thought yesterday. Thanks for this exchange!

Are there any suggestions in the literature as to how myrmecophay (sp.?) arises in various groups? It’s such a specialized lifestyle and, in some animals (like the aardwolf), I can’t imagine an “in-between” feeding strategy. That’s not to say one didn’t exist (it must have) but I’m just curious as to what the vector toward termite-eating might be.

I don’t think anyone has talked about it seriously, but my assumption would be that they get there via some less specialised form of insertivory, or by taking termites as part of the diet and then specialising to collect them more efficiently and dealing with the formic acid / excess dirt / walls of the nest etc. as appropriate. Certainly thinks like armamdillos can and do eat ants without being quite so dedicated as others and have some exaptations in being good diggers, a tough skin and long-ish snout.